The present invention relates to improvements in the electron microscope, electron beam lithography apparatus, ion microscope, secondary ion mass spectrometer or other charged particle beam apparatus employing a finely focused charged particle beam (electron or ion beam). Still more particularly, the invention relates to structural improvements in a charged particle beam apparatus that uses an electrostatic lens for finely focusing the charged particle beam.
Conventionally, the above-mentioned charged particle beam apparatus utilizes a magnetic lens to finely focus the charged particle beam onto a sample for observation or processing. Because it contains an exciting coil, the magnetic lens is difficult to reduce in size. Thus there is a limit to downsizing the charged particle beam apparatus using the magnetic lens.
There have been attempts to reduce the entire size of the charged particle beam apparatus using an electrostatic lens for focusing. In such cases, the electrostatic lens is typically an Einzel lens 23 shown in FIGS. 6(A) and 6(B). This Einzel lens comprises three vertically stacked electrodes. The upper and lower electrodes are generally connected to ground potential, while the middle electrode is supplied with a negative (or positive) lens voltage V.sub.L1. This generates the necessary lens action. In the charged particle beam apparatus utilizing the Einzel lens 23, a deflector 9 is located within a ground potential space above the lens (i.e., on the side of a charged particle source 1) or below it (on the side of a sample 8). (This aspect of the charged particle beam apparatus is illustratively disclosed in Japanese Patent Publication No. 63-67743.)
With a charged particle beam apparatus that finely focuses a charged particle beam onto a sample, the spatial resolution of the apparatus is determined by the diameter of a charged particle beam spot on the sample surface. To obtain the smallest possible spot requires minimizing the optical aberration of a charged particle beam optical system for focusing which is used in the charged particle beam apparatus. If the charged particle beam optical system were constituted by an electrostatic lens arrangement alone, the size of the apparatus would be reduced as a whole. However, the conventional Einzel lens that has been used as the electrostatic objective lens cannot sufficiently raise the spatial resolution because its optical aberration is higher that of the magnetic objective lens. For this reason, using the Einzel lens as the objective lens cannot downsize the entire apparatus without degrading the performance thereof.
In a charged particle beam apparatus that finely focuses a charged particle beam onto the sample surface for observation or processing, a field emission type charged particle source is used to enhance the resolution of such observation or processing.
With the above-mentioned field emission type charged particle source, performing stable field emission of the charged particle beam requires keeping the internal space of the source in the highest possible vacuum state. Thus the charged particle beam apparatus employing the field emission type charged particle source uses two separate vacuum pumps. One pump evacuates a sample chamber in the apparatus, and the other pump evacuates the charged particle source. The dual pump structure makes it possible to evacuate the sample chamber and the charged particle source at different levels of negative pressure. At least when the field emission type charged particle source is emitting a charged particle beam, the source is directly evacuated to a high degree of vacuum by its high vacuum pump independent of the vacuum pump for the sample chamber. This keeps the charged particle source in a high vacuum state regardless of the degree of vacuum in the sample chamber.
Where the charged particle source is evacuated from the atmospheric pressure state it is in or is baked to be gassed, the source is evacuated to a low degree of vacuum. In this case, the high vacuum pump mentioned above is not appropriate for low vacuum evacuation; it must be taken over by yet another pump, i.e., a low vacuum pump that operates for relatively low degrees of vacuum.
Switching between the low and high vacuum pumps for proper evacuation of the charged particle source is generally accomplished using a vacuum pump switching valve and its associated piping. These parts are usually located alongside a beam column containing the charged particle source and the charged particle beam optical system. When the charged particle source is to be evacuated from a low-vacuum state or subjected to degassing, the low vacuum pump is first connected to the source. The low vacuum pump then proceeds to evacuate the charged particle source to a low degree of vacuum. When the charged particle source has attained a certain degree of vacuum or has completed degassing through operation of the low vacuum pump, the switching valve is operated to select the high vacuum pump. The high vacuum pump then starts evacuating the charge particle source to a high degree of vacuum. (In this case, a negative pressure differential exists between the charged particle source and the sample chamber). This type of differential evacuation system is illustratively described in Journal of Electron Microscopy, Vol. 22, No. 2 (1973), pp. 141-147.
On the other hand, it is known that the beam column in the charged particle beam apparatus can be made very small and lightweight if the internal optical system for focusing the beam contains an electrostatic lens arrangement alone. With such a charged particle beam apparatus containing a small and lightweight beam column, it is not desirable to install a vacuum pump switching valve or its associated piping alongside the beam column. Since the switching value and its pipes are difficult to downsize, they are often dimensionally incompatible with the small-sized beam column, and hence with the smaller charged particle beam apparatus that comprises the column. Even if the vacuum pump switching valve and its related piping were somehow made smaller with considerable difficulty, the evacuation conductance of the pumping system for evacuating the charged particle source would be drastically lowered.